CRYSTAL RIVER, Fla. — “Hello, Beautiful.” That’s how Ross Files, “Captain Ross,” greets a massive West Indian manatee in the Crystal River National Wildlife Refuge in King’s Bay on Florida’s west coast.

Beauty is, indeed, in the eye of the beholder.

The sea cow has lumpy gray skin and a saggy snout covered in bristly whiskers. Tiny eyes are sunken into a potato-like head and her 1,500-pound body is shaped like a blimp.

In Crystal River, you don’t have to be pretty to be popular.

People around here can’t get enough of these sluggish gentle giants, and they know observing them up-close is a privilege. There’s nowhere else in North America where humans can legally swim and interact with this endangered species.

This leviathan is clearly pleased to see her old friend and flips over for a belly rub like an eager-to-please golden retriever. Files talks to her in a soothing voice, and she seems to listen. The pair shares a rare species-to-species connection.

Files introduces her to his pals — guests staying at the Plantation on Crystal River, a resort 80 miles north of Tampa famous for its swim-with-manatee tours. There is a palpable undercurrent of excitement as Files allows the group to gently touch the inquisitive marine mammal — with one hand only as instructed in the pre-tour orientation.

Soon, dark shapes materialize beneath the calm surface. At least a dozen manatees join the party, stirring up the water with their paddle-shaped tails until it’s a murky brown, rendering everyone’s snorkeling masks useless. The docile creatures are as curious and playful as dolphins — but don’t expect any aerial acrobatics from these languid behemoths.

If the manatees tire of their human admirers, they can swim to a roped-off area that’s off-limits to swimmers.

King’s Bay is home to the largest concentration of manatees in the world. The warm, spring-fed water and an abundance of sea grass, a staple of the herbivores’ diet, make it the ideal habitat.

Manatees may look a bit like whales, but they are not related to them and don’t have a protective layer of blubber to insulate them from the cold. When the water temperature in the Gulf of Mexico falls below 68 degrees, they flock to their cozy winter refuge in King’s Bay, which is 72 degrees year round, thanks to a continuous flow of warm water from dozens of natural springs.

If you plan to take a tour in the summer, don’t worry. There are more manatees to observe in the colder months, but they are regularly spotted in King’s Bay year round.

A tour highlight is snorkeling in the pristine emerald waters of Three Sisters Spring. Files refers to it as “the bedroom” because this is where the manatees go to get a little shut eye. The water is as transparent as glass, so it’s easy to view the resting animals as they lay as motionless as giant boulders.

People often have emotional responses to being so close to these fascinating creatures. Some shriek with delight; others are moved to tears.

Nobody understands these reactions better than Files. The retired Maine lobster fisherman says his first manatee encounter was life changing. What started as a Florida vacation resulted in a move across the country.

“I had never even heard of a manatee until we did a tour,” Files says. “I just fell in love with them. I said, ‘this is awesome,’ and I packed up and moved down.”

As a tour guide, Files not only gets to spend time with his beloved manatees every day, he also educates visitors about their plight.

While surrounded by dozens of manatees, it may be hard to comprehend that they are endangered, but habitat loss and collisions with watercraft are a constant threat. Manatees are mammals that must surface to breathe, and the slow moving creatures don’t stand a chance when faced with a speeding boat. Many bear scars from propellers. (Strict speed limits are enforced in King’s Bay) Other threats come from becoming entangled in crab trap lines, getting trapped in canal locks and ingesting fishing gear and litter.

As of 2012, the endangered manatee population in King’s Bay had rebounded from about 30 in the 1960s to approximately 600. Statewide, the population climbed to about 5,000, but last year a mysterious ailment and a surge of red tide algae caused more than 800 manatee deaths, about 16 percent of Florida’s manatee population, according to the Florida Fish and Wildlife Institute in St. Petersburg.

Because the manatee population is vulnerable, the “swim with” program has its critics. There are conservationists who disapprove of humans having such close contact with an endangered, federally protected species, but Files says the program is key to understanding their need for protection.

“We try very hard to make customers aware this is not a petting zoo,” Files says. Touching is permitted, but there are strict rules. “We strive to make this an educational tour and teach respect for this wonderful creature.”